1. Field of the Invention
The present invention relates to an acousto-optic variable-wavelength TE/TM mode converter, and a variable-wavelength optical filter using this converter.
A TE/TM mode converter of this kind can convert TE mode light into TM mode light or vice versa by using an acousto-optic interaction. Accordingly, this converter is available for various types of optical devices or optical systems. For example, a variable-wavelength optical filter can be configured by combining this TE/TM mode converter and polarizers. Such a variable-wavelength optical filter is available, for example, as a variable-wavelength optical filter for use in a variable-wavelength coupler/splitter, etc. in a wavelength division multiplexing (WDM) optical communications system.
2. Description of the Related Art
An acousto-optic TE/TM mode converter is conventionally known. This converter is normally composed of an area where a surface acoustic wave (SAW) is excited and propagated, and an optical waveguide that is arranged to acousto-optically interact with the surface acoustic wave, which are arranged in a piezoelectric substrate.
The surface acoustic wave is excited by applying a high-frequency (RF) electric signal to a comb electrode arranged in a substrate, and propagated while being guided by a coupled acoustic waveguide arranged on the substrate. By arranging an optical waveguide in the propagation area of the surface acoustic wave, light and the surface acoustic wave are made to interact with each other. Consequently, only the light having the wavelength that phase-matches the wavelength of the surface acoustic wave is selectively TE/TM-mode-converted. The wavelength of the surface acoustic wave is changed by varying the frequency of the high-frequency signal applied to the comb electrode, so that the wavelength of TE/TM-mode-converted light can be selected. As references for such a TE/TM mode converter, for example, xe2x80x9cOptical Integrated Circuitxe2x80x9d(written by H. Nishihara (et al.), published by Ohmsha), xe2x80x9cFundamentals of Optical Electronicsxe2x80x9d written by A. Yariv, published by Maruzen Co, Ltd., etc. can be cited. For further details, please see these references.
The above described TE/TM mode converter is applicable to various optical devices with its mode conversion capability. For example, a variable-wavelength optical filter can be formed by respectively arranging polarizers in an input portion and an output portion of the optical waveguide in the above described TE/TM mode converter. In this case, the variable-wavelength optical filter can be configured as a bandpass filter or a bandstop filter (rejection filter) by changing the arrangement, etc. of the two polarizers. Thus configured variable-wavelength optical filter can simultaneously filter multiple wavelengths by applying a plurality of high-frequency electric signals to comb electrodes in a TE/TM mode converter. Namely, the lights having the wavelengths that correspond to the frequencies of the applied high-frequency electric signals can be filtered at the same time.
However, a side lobe, which is undesirable as an optical filter characteristic, occurs in such type of a variable-wavelength optical filter. As a result, a wavelength other than a selected wavelength is filtered, or the flatness of the filtering characteristic is deteriorated.
To overcome the above described problems, for example, the technique with which the intensity distribution of a surface acoustic wave is given in the longitudinal direction of an optical waveguide, which is disclosed by U.S. Pat. No. 5,400,171, is adopted. With this technique, however, a selected wavelength gets out of position or the depth of rejection is deteriorated if high-frequency electric signals having adjacent frequencies are simultaneously applied to the same acousto-optic interaction area. Accordingly, lights having multiple adjacent wavelengths cannot selectively be filtered, leading to a difficulty in use of such an optical filter for a variable-length coupler/splitter in a WDM optical communications system.
In the meantime, for example, the technique with which an interaction area in a TE/TM mode converter is separated into two areas, and high-frequency electric signals having adjacent frequencies are distributed into the respective areas may be also considered as disclosed by U.S. Pat. No. 5,455,877. The top view of the configuration of the TE/TM mode converter adopting such a technique is shown in FIG. 1A.
The TE/TM mode converter shown in this figure comprises: an optical waveguide 1; a coupled acoustic waveguide 2 composed of adjacent first and second acoustic waveguides 2a and 2b; first and second comb electrodes 3 and 4 respectively arranged at both ends of the coupled acoustic waveguide 2; and a surface acoustic wave absorber 5 arranged to go across the center of the coupled acoustic waveguide 2. The coupled acoustic waveguide 2 is partitioned off by three areas 7a, 7b, and 7c, which are titanium(Ti)-diffused in a piezoelectric substrate. The central Ti-diffused area 7b is a gap (a gap between acoustic waveguides) that separates the two acoustic waveguides 2a and 2b. The comb electrodes 3 and 4 are arranged within the first acoustic waveguide 2a, whereas the optical waveguide 1 is arranged within the second acoustic waveguide 2b.
In the TE/TM mode converter having such a configuration, if high-frequency electric signals having adjacent frequencies are respectively applied to the comb electrodes 3 and 4, surface acoustic waves W1 and W2, which correspond to the respective frequencies, occur on the surface of the piezoelectric substrate. These surface acoustic waves W1 and W2 propagate on the surface of the board while being guided by the coupled acoustic waveguide 2, and is finally absorbed by the absorber 5. Each of the surface acoustic waves W1 and W2 acousto-optically interacts with the light propagating through the optical waveguide 1 during this propagation. Only the lights having the wavelengths that respectively correspond to the frequencies of the above described high-frequency electric signals are selectively TE/TM-mode-converted and output from the output portion of the optical waveguide 1. In this case, the intensity distribution of the surface acoustic waves W1 and W2 in the longitudinal direction of the acoustic waveguide 2 are those shown in FIG. 1B. Substantially, the area where the surface acoustic waves W1 and W2interact with light are separated into two.
Accordingly, if an optical filter is configured by using the TE/TM mode converter having such a configuration, the above described problems that the selected wavelength gets out of position, and the depth of rejection is deteriorated are overcome, whereby lights having adjacent wavelengths can selectively be filtered at the same time.
However, with the TE/TM mode converter having the configuration shown in FIG. 1A, the area where the surface acoustic waves W1 and W2 interact with light are completely separated into two in the longitudinal direction of the acoustic waveguide 2. Therefore, the interaction length for each interaction area is shortened, and a sufficient interaction is difficult to be obtained as it is. Therefore, the power of a high-frequency signal must inevitably be intensified to secure a sufficient interaction, and at the same time, a side lobe characteristic is deteriorated.
An object of the present invention is to provide an acousto-optic variable-wavelength TE/TM mode converter which can selectively mode-convert lights having multiple adjacent waves at the same time while preventing the power of a high-frequency signal from increasing so as to overcome the conventional problems described earlier.
Another object of the present invention is to provide a variable-wavelength optical filter which can selectively filter lights having multiple adjacent wavelengths at the same time while preventing the power of a high-frequency electric signal from increasing.
The present invention is configured as follows to achieve the above described objects.
An acousto-optic variable-wavelength TE/TM mode converter according to the present invention comprises: an optical waveguide arranged in a piezoelectric substrate; a plurality of comb electrodes generating surface acoustic waves in the piezoelectric substrate; a coupled acoustic waveguide guiding the surface acoustic waves propagating in the piezoelectric substrate; and surface acoustic wave absorbers absorbing the surface acoustic waves. A gap between acoustic waveguides, which is arranged between the two acoustic waveguides configuring the above described coupled acoustic waveguide, is comprised at least in a partial area. The above described optical waveguide is arranged in one of the two acoustic waveguides separated by the gap between the acoustic waveguides, whereas the two comb electrodes are arranged in the other of the acoustic waveguides. Additionally, the respective surface acoustic wave absorbers are arranged at the positions which are interposed between the two comb electrodes, and are different in correspondence with the respective two comb electrodes.
With such a configuration, respective surface acoustic absorbers are arranged at the positions which are interposed between two comb electrodes, and are different in correspondence with the respective two comb electrodes. As a result, an interaction area exists for each surface acoustic wave, and the locations of these two interaction areas do not match. Therefore, even if a plurality of high-frequency electric signals having adjacent frequencies are applied to the comb electrodes, the problems that a selected wavelength gets out of position, and the depth of rejection is deteriorated do not arise.
Besides, unlike the configuration shown in FIG. 1A, in which a single surface acoustic wave absorber is arranged to completely separate the interaction area into two, respective surface acoustic wave absorbers are arranged at different positions corresponding to two comb electrodes. Therefore, an interaction area is not completely separated into two. As a result, the interaction length of each interaction area can sufficiently be secured, thereby eliminating the need for applying high-frequency electric signals having high power to the comb electrodes, and also preventing a side lobe characteristic from being deteriorated.
It is desirable to configure the TE/TM mode converter as follows, if the above described two comb electrodes are respectively defined to be first and second comb electrodes, the above described surface acoustic wave absorbers, which respectively correspond to the first and second comb electrodes, are defined to be first and second surface acoustic wave absorbers. Namely, the TE/TM mode converter desirably has a configuration such that: the first comb electrode, the second surface acoustic wave absorber, the first surface acoustic wave absorber, and the second comb electrode are arranged in this order in the other of the acoustic waveguides stated earlier; a first surface acoustic wave generated by the first comb electrode is absorbed by the first surface acoustic wave absorber, whereas a second surface acoustic wave generated by the second comb electrode is absorbed by the second surface acoustic wave absorber; and the first and second surface acoustic waves respectively have intensity distributions where an intensity is high in a middle portion of the optical waveguide, and low at both ends of the optical waveguide, and the peak positions of the intensity distributions are different.
Additionally, if the TE/TM mode converter according to the present invention is used in a WDM optical communications system, etc., it is desirable to respectively assign high-frequency electric signals having adjacent frequencies to the above described two comb electrodes. In this way, the frequency interval of high-frequency electric signals applied to the same comb electrode can be widened, so that a more satisfactory wavelength selection characteristic can be expected.
A variable-wavelength optical filter according to the present invention is configured by comprising: an acousto-optic variable-wavelength TE/TM mode converter configured as described above according to the present invention; and first and second polarizers respectively arranged at the input and output portions of the acousto-optic variable-wavelength TE/TM mode converter.
By using the acousto-optic variable-wavelength TE/TM mode converter according to the present invention as described above, a variable-wavelength optical filter which can selectively filter lights having multiple adjacent wavelengths at the same time while preventing the power of high-frequency electric signals from increasing.
Here, if the first and second polarizers are arranged to make their axes parallel, a bandstop filter is configured. Or, if the first and second polarizers are arranged to make their axes orthogonal, a bandpass filter is configured.
In the TE/TM mode converter according to the present invention, as long as respective surface acoustic waves, which are generated by a plurality of comb electrodes and propagate in a piezoelectric substrate, have the following intensity distribution regardless of the arrangement of an optical waveguide, acoustic waveguides, comb electrodes, surface acoustic wave absorbers, etc., a similar action can be expected. That is, the intensity distributions of respective surface acoustic waves may be distributions where an intensity is high in a middle portion of the optical waveguide, and low at both ends of the optical guide, and the peak positions of the intensities are different, and the distributions partially overlap in an area where the intensities are low.